Psychophysical measurement of the foveal avascular zone

The size of the foveal avascular zone (FAZ) reflects the condition of the microcapillary circulation in the foveal area. A number of studies have shown a significant increase in FAZ size in conditions of vascular pathology including diabetes. A non-invasive method of estimating FAZ size may be a valuable addition to a psychophysical test battery. We used a blue-light field to reveal the activity of the retinal leucocytes to two groups of normal subjects with average ages 24 and 61 years. Two methods were used to estimate the FAZ size, making use of the fact that subjects could see their own FAZ as the area free of leucocyte activity. The first method required subjects to indicate when leucocyte activity stopped as the field size was decreased and when it reappeared as field size was increased. The second required subjects to match the size of the leucocyte-free area with a white circle superimposed on the field. A search algorithm was used to change circle size according to subjects' responses. Results show a significant difference in FAZ size between the two age groups with both methods of measurement, and estimates were comparable to those reported using angiographic methods.